The present invention relates to a washer type pressure sensor which is incorporated between portions for detecting the pressure between the portions.
As shown in FIGS. 1 and 2, a conventional washer type pressure sensor 1 is formed by stacking a plurality of plain ring members which comprises pressure receiving plates 3 and 5, piezoelectric elements 7 and 9, and an electrode plate 11. The openings 13 of the ring members are in alignment with each other. The electrode plate 11 has an output terminal 12 extending from a part of the external periphery thereof and is sandwiched between the piezoelectric elements 7 and 9. Each of the pressure recieving plates 3 and 5 is disposed on the outside surface of the piezoelectric elements 7 and 9 in close contact therewith. The inner and outer peripheries of such plurality of plain ring members are molded with insulation adhesives 15 and 17 made of plastic or the like, so that all the plain ring members are integrally formed in a washer-shape, as shown in FIGS. 2, 3 and 4.
The piezoelectric elements 7 and 9 are disposed so that the polarization directions of both the elements are opposed to each other, and the pressure receiving plates 3 and 5 have an different pole from a pole of the electrode plate 11. Accordingly, the insulation adhesives 15 and 17 serve to insulate between the pressure plate 3 and 5 and the electrode plate 11.
As shown in FIG. 5, the prior art pressure sensor 1 described above is often attached to an opening 19 provided at a cylinder head 21 of an internal combustion engine for inserting an ignition plug 23. The sensor 1 is held between a projection 23c of the plug 23 and the cylinder head 21 so that the sensor 1 can detect a change of a force for fastening the ignition plug 23, when the fuel is burned, to determine the pressure in a combustion chamber 25.
However, in the prior art pressure sensor 1, the insulation adhesive 15 molding the inner periphery thereof is made of a synthetic resin material, such as a plastic, which is relatively fragile against a shock. Therefore, when the ignition plug 23 is engaged with the opening 19 to which the pressure sensor is attached, an outside periphery 23a at the tip of the plug 23 or threads 23b may hit against the inner periphery of the pressure sensor 1 and injure the insulation adhesive 9 unless the plug 23 is carefully and exactly inserted. This causes the insulation between the pressure receiving plates 3 and 5 and the electrode plate 11 to be destroyed, and, as a result, an electrical short-circuit between the pressure plates 3 and 5 and the electrode plate 11 appears.